This invention pertains to a process for purifying p-phenylenediamine-type color developers used in photographic color development and reproduction applications. More particularly, the present invention relates to a process for purifying p-phenylenediamine-type photographic color developers in their free-base that are substantially free of sulfate ions. This invention permits the preparation of useful solid and solution compositions of p-phenylenediamine color developers in the free-base form. In another embodiment of the present invention, the method provides an improved process for preparing p-phenylenediamine color developers in the acid salt form.
The basic processes for obtaining useful color images from exposed color photographic silver halide materials include several steps of photochemical processing such as color development, silver bleaching, silver halide fixing and water washing or dye image stabilizing using appropriate photochemical compositions.
The photofinishing industry is comprised of three primary segments: the manufacture of photographic chemicals to supply photochemical formulators, the formulation of these chemicals into useful compositions for photochemical processing, and the processing of sensitized photographic color elements. Formulated photographic processing (photofinishing) solutions provided to the processors are complex, multi-part, multi-component mixtures, the specific compositions of which vary significantly according to the intended use and the formulator.
The types of materials which are ultimately mixed by a photofinisher just prior to use and delivered to the photo processing bath include: solubilizing agents, e.g., organic co-solvents; bases for pH control; color developing agents or color developers; preservatives; antifoggants; sequestering agents; buffering agents; clarifying agents; stain-reducing agents; anti-bacterial or anti-fungal agents; surfactants; and other function-specific materials.
In conventional silver halide color photography, photofinishing solutions in the final processing bath includes a color developer, a preservative, and a solvent or combination of solvents. The color developer agent, in its free base form, serves to amplify the latent image on the silver halide crystals by reacting, in its oxidized form, with a dye-forming color coupler and subsequently, produce a color image of the exposed, sensitized photographic color element.
Suitable color developing agents that are known in the art include aromatic amino color developing agents such as p-phenylenediamines, and particularly the N,N-dialkyl-p-phenylenediamines in which the alkyl groups or the aromatic nucleus can be substituted or unsubstituted. The p-phenylenediamine color developers most commonly present in photofinishing compositions include: N,N-diethyl-2-methyl-p-phenylenediamine monohydrochloride (CD-2); N-ethyl-N-2-(methanesulfonylaminoethyl)-2-methyl-p-phenylenediamine sesquisulfate monohydrate (CD-3); and N-ethyl-N-(2-hydroxyethyl)-2-methyl-p-phenylenediamine sulfate (CD-4), listed here in the commonly used acid salt form.
Color developing compositions are commonly supplied in multiple parts or solutions because of the requirement to separate components which are reactive or unstable over extended periods of time. One part normally includes an acid salt of a p-phenylenediamine color developer, typically in the form of a stabilized aqueous solution. These solutions are acidic since the color developer is formulated in this part as the acid salt. Another part typically contains a base or combination of bases which, upon mixing of the parts, serves to neutralize the acid associated with the part containing the color developer and to establish the desired alkalinity of the mixed color developing composition. These solutions are typically mixed immediately before use. Upon combination of all parts and water, a homogeneous color developing composition can usually be obtained for the working strength solution in the processing machine.
There is a desire in the industry to reduce the number of parts used to prepare color developing compositions, and particularly to prepare replenishing solutions. A wide range of compositions are described in the art or commercially available as xe2x80x9cready to usexe2x80x9d solutions, concentrates or dry formulations. Liquid concentrates have only to be diluted with water to provide a working strength-solution. Dry formulations need only be dissolved in water.
The free-base forms of these color developers are presently industrially processed in solutions as non-isolated intermediates prepared from the acid salt form of the p-phenylenediamine. The free base form of the color developer is prepared by mixing an aqueous solution of the color developer salt with sodium hydroxide. For example, U.S. Pat. No. 6,017,687 issued to Darmon et al. on Jan. 25, 2000 and U.S. Pat. No. 6,077,651 issued to Darmon et al. on Jun. 20, 2000 disclose a homogeneous, single-part, chloride ion-free color developing concentrate that is prepared using a critical sequence of steps. In the first step, an aqueous solution of the color developing agent is prepared typically from the sulfate salt and an alkali metal base to provide a stoichiometric proportion of alkali metal ions to render the color developer in a free base form. A disadvantage of obtaining the free base color developer in this manner is that an insoluble sulfate salt that precipitates from the solution. The solid precipitate must then be removed from the solution in an extra step. Additionally, due to the solubility of the sulfate salt there will always be some residual sulfate ions in solution, which these patents teach is detrimental to the composition stability of a photo finishing solution.
The use of such combined photofinishing solutions results in simplicity of operation with a reduction in the potential of mixing errors and poor photoprocessing results. The major incompatibility of the traditional parts of photofinishing solutions is that between the acidic aqueous part, containing the acid salt of the p-phenylenediamine color developer, and the alkaline part, containing the base or combination of bases. Since the active color developer in the final, mixed photofinishing composition is the free base form of the color developer, full consolidation of parts to produce a single-part formulation requires a source of the p-phenylenediamine color developer in free base form as the necessary ingredient. As used herein, the term xe2x80x9cfree-basexe2x80x9d refers to phenylenediamine compounds devoid, or essentially devoid, of acid addition salts of phenylenediamines.
The preparation of N,N-disubstituted p-phenylenediamines has been known for many years; the earliest and most complete summary of the preparation of these compounds is found in the Journal of the American Chemical Society (Bent, Dessloch, Fassett, James, Ruby, Sterner, Vittum, and Weissberger, JACS 73, 3100-3125, 1951). For the synthesis of each of 61 N,N-disubstituted p-phenylenediamines, the last step was a reduction of either a nitroso, nitro, or an azo precursor. These nitroso, nitro, or azo precursors were subsequently reduced to the desired N,N-disubstituted p-phenylenediamines (Free Bases) under the conditions of zinc and hydrochloric acid in water or by catalytic hydrogenation in ethyl alcohol. Generally, for the purposes of the present invention, the p-phenylenediamine free bases are prepared by the hydrogenation of a water-wet nitro or nitroso precursor in an alcohol solution in the presence of a heterogeneous hydrogenation catalyst under hydrogenation conditions of temperature and pressure. The heterogeneous catalyst is separated from the solution, providing p-phenylenediamine free base solutions with typical composition ranges of 25-40 weight percent p-phenylenediamine free base, 40-50 weight percent of the alcohol solvent, 15-25 weight percent water, and 2-6 percent (by difference) of total impurities. These intermediate free bases were purified by either crystallization or distillation, then converted to their more stable acid salt form.
Manufacturers of p-phenylenediamine color developers have avoided the use of traditional batch distillation process technology of the free base form due to oxidative and thermal degradation. Degradation by-products have been found to impart adverse effects on the photographic quality, rendering such distilled products unfit as a photographic color developer. A secondary factor in avoiding conventional distillation has been the low process yield.
Recovery of the p-phenylenediamine color developers by crystallization is also not without its problems. The resulting solution of the free base color developer is generally purified by contacting the solution with a decolorizing agent, such as activated carbon, which results in significant product yield losses to the decolorizing agent. Following decolorization, an appropriate mineral acid is added to the solution to convert the free base to its respective acid salt. The solution is cooled and the acid salt of the color developer is isolated by filtration. Although the filtrate typically contains a significant amount of the desired product, it is usually discarded due to the high concentration of impurities in the stream. In addition, the presence of water in the filtrate increases the solubility of the color developer acid salt and contributes to further yield losses to the filtrate. Typically, the isolated acid salt precipitate undergoes a re-crystallization step to achieve acceptable purity. Re-crystallization can be performed in an alcohol or ether solvent with a diminished yield loss, compared to the initial crystallization. The filtrate from this second crystallization is generally pure enough to permit the recycle of all or a significant portion of it. Accordingly, in current industry practice, there are significant yield losses which negatively impact the process economics and purity of the free base precursor to the acid salt.
There is a need for a process whereby direct purification and isolation of the color developer free base can be achieved with acceptable purity and which overcomes the yield losses and additional costs associated with current process technology.
The present invention provides a process for the purification of p-phenylenediamine color developers in their free-base form. The process includes the steps of subjecting a solution containing said free base color developer to short path distillation having a temperature less than about 250xc2x0 C. and a pressure of less than about 10 mm of mercury and wherein said short path distillation includes at least one of falling film evaporator, thin film evaporator, wiped film evaporator, or short-path evaporator.
Yet another aspect of the present invention is an improved method for preparing an acid salt from the p-phenylenediamine free base color developer. The method includes the steps of: a) dissolving a purified p-phenylenediamine free base distillate in an organic solvent; b) contacting the dissolved free base distillate with an appropriate mineral acid; and c) crystallizing the color developer in the acid salt form.
It is an object of the present invention to provide a process for purifying p-phenylenediamine color developers in their free-base form.
It is another object of the present invention to provide an improved method for preparing an acid salt of a p-phenylenediamine color developer.
These and other objects and advantages of the present invention will become more apparent to those skilled in the art in view of the following description and the accompanying drawing wherein like parts and objects have similar reference numerals. It is to be understood that the inventive concept is not to be considered limited to the constructions disclosed herein but instead by the scope of the appended claims.